I personally use 6mm cord and I carry various lengths of prussik as they are useful in different situations. I pretty good length is roughly 25cm loop which translates to 60cm of cord. Just make sure that you buy very supple cord, it bites much better and suffers less from twisting.

Just don't go trad climbing without them! If you do make sure that your partner is either awesome and can hoist you or can call rescue

I bought some 4mm cord and it performed brilliantly on many descents from limestone routes in the South West of England, this weekend past - including one good session of hanging about, extracting stuck stoppers. It does bite much more effectively than my old 6mm ones used to.

For the record, 60cm is well too short, even for skinny ropes, particularly if you want to tie a French prusik.

I used one metre per loop and made them both the same length - they aren't likely to be used for anything that would require them to sit, nicely, next to each other.

Nic: We did some investigation but ruled it out - one can't really get anywhere in a hurry with only a bicycle and trains.

(One can have a lot of fun going places very, very slowly, with a bicycle, but that is a different matter entirely.)

We are currently gloating about our budget trad. outing: the fact that we achieved two good climbing days without requiring leave, for under £200 all up, is honestly incredible. When you aren't getting much, you take what you can get, I suppose.